Physiological and pharmacological basis of GABA and glycine action neurons of mudpuppy retina. I. Receptors, horizontal cells, bipolars, and G-cells

Intracellular recordings from identified and unidentified neurons of the mudpuppy retina were maintained in a perfused retina-eyecup preparation. Putative transmitters γ-aminobutyric acid (GABA) and glycine and the antagonists picrotoxin, bicuculline, and strychnine were added to the bathing medium while monitoring input resistance, membrane potential, and light-evoked response properties of retinal neurons. GABA and glycine in concentrations up to 10 mM did not effect photoreceptors; strychnine, bicuculline, and picrotoxin did not influence photoreceptor wave form. Horizontal cells were depolarized if the GABA and glycine concentration exceeded 2 mM; this effect was either absent or required a much larger dose if synaptic transmission was eliminated with cobalt. Depolarizing bipolars were more GABA than glycine sensitive while hyperpolarizing bipolars were comparatively more glycine sensitive. Depolarizing bipolars were enhanced by GABA antagonists; hyperpolarizing bipolars were enhanced by strychnine. These observations raise the possibility that bipolar cells are tonically affected by dark active GABA- and glycinergic pathways. Center-surround antagonism of depolarizing and hyperpolarizing bipolars was not blocked by picrotoxin, bicuculline, or strychnine. Some changes in spatial-response parameters were seen in depolarizing bipolars when picrotoxin or bicuculline was applied. An anatomically unidentified neuron (the G-cell) was depolarized by GABA at doses of 1 mM or less; GABA depolarization was enhanced after block of light-evoked activity by Co²⁺. G-cells were hyperpolarized and showed larger light responses by picrotoxin and bicuculline application but not by strychnine. The mechanism of response enhancement may result from a block of dark-released GABA, which maintains G-cells in a slightly depolarized state. The observations of this study suggest that both GABA- and glycinergic pathways play a role in integrative neuronal activity and that GABA may be associated with the on pathway while glycine may be intimately related to the off channel. It is argued that the amacrine cells are the most likely GABA- and glycinergic neurons. Since antagonistic center-surround organization has been attributed to horizontal cell function, the findings do not support the idea that horizontal cells are either GABA- or glycinergic.